Good visuals are critical to building an attractive website that will entice shoppers to stop and take a look. There is a lot of free clip art out there, but many times it looks, well, cheap. And if visitors see the same graphic on your site and several others, that doesn't give them the best impression about any of the sites. If possible, create some of your own graphic icons and images using a good illustration program like Adobe Illustrator or Corel Paint Shop Pro.
With an illustration program you can create vector images. These, as opposed to bitmap images, are editable, scalable and usually small in file size. While bitmap images can be edited, you need an image-editing program like Adobe Photoshop in order to change individual pixels or add filters or special effects. But the image's size can't be increased very much without loss of image quality and a big increase in file size.
Another option is to purchase some higher quality clip art. Getty Images' PhotoDisc offers high quality photo images and artwork that isn't always that expensive, particularly for the lower resolution images you would need for your website.